Handle special characters in JSON model dump. (#9474)

tests/python/test_basic_models.py

@@ -439,6 +439,26 @@ class TestModels:
                       'objective': 'multi:softmax'}
         validate_model(parameters)
 
+    def test_special_model_dump_characters(self):
+        params = {"objective": "reg:squarederror", "max_depth": 3}
+        feature_names = ['"feature 0"', "\tfeature\n1", "feature 2"]
+        X, y, w = tm.make_regression(n_samples=128, n_features=3, use_cupy=False)
+        Xy = xgb.DMatrix(X, label=y, feature_names=feature_names)
+        booster = xgb.train(params, Xy, num_boost_round=3)
+        json_dump = booster.get_dump(dump_format="json")
+        assert len(json_dump) == 3
+
+        def validate(obj: dict) -> None:
+            for k, v in obj.items():
+                if k == "split":
+                    assert v in feature_names
+                elif isinstance(v, dict):
+                    validate(v)
+
+        for j_tree in json_dump:
+            loaded = json.loads(j_tree)
+            validate(loaded)
+
     def test_categorical_model_io(self):
         X, y = tm.make_categorical(256, 16, 71, False)
         Xy = xgb.DMatrix(X, y, enable_categorical=True)